1. Field of the Invention
This invention relates generally to emulsion-treating (dehydration) apparatus, and, more particularly, to an improved energy-conserving electrostatic coalescer for separating emulsified brine from oil during longitudinal flow through a horizontally elongated metal tank.
2. Description of Related Art
It is well known that petroleum as it is naturally produced from an underground formation must be treated so as to separate and remove entrained gas and emulsified brine or water, in order to render the oil pipelineable. Various techniques and processes have heretofore been employed in order to minimize treatment time and avoid high-energy consumption. In particular, U.S. Pat. No. 4,329,159 (xe2x80x9c""159xe2x80x9d) to Bull, entitled, xe2x80x9cEnergy Saving Heavy Crude Oil Emulsion Treating Method and Apparatus for Use Therewithxe2x80x9d describes a method and apparatus comprising an elongated horizontal cylindrical tank, divided by internal partitions, into compartments through which the petroleum will sequentially flow. Burner-fired heaters are normally included in an upstream heater section for heating the emulsion to a desired temperature, during which most of the entrained gas and some of the brine will separate from the emulsion. The partially de-emulsified brine then flows into a coalescing section, encountering a series of baffles adapted to encourage even flow of fluids and to avoid the formation of flow channels within the fluid body. Additionally, high-potential electrostatic fields are applied by energizing grids with high voltage potential. The grids are adjacent to each grounded baffle, which creates the fields between each grid and grounded baffle. The resultant electrostatic fields coalesce the droplets of brine remaining in the oil into drops of sufficient size and weight that they flow downwardly by gravity to the bottom of the coalescing section for removal. Oil substantially free of brine, then flows over a wall into a reservoir where the brine-free oil may be intermittently or continuously discharged, without affecting the liquid level in the treater.
While the use of the baffles and the electrostatic elements in the ""159 patent have resulted in enhanced separation of the oil and brine, the invention herein described discloses further improvements in which the metallic apertured grids have been expanded so as to take up more of the flow area, and include longitudinally-extending sections or wings that provide a more even and larger electrostatic zone between the grid edges and the wall of the vessel (grounded) to improve the coalescence of the brine within the emulsion flowing through the treater.
In addition, to further improve and enhance the separation of the oil and brine, the present invention includes the use of multiple transformers. After conducting a number of experiments, it was found that wet emulsion is much more conductive than lean (dry) emulsion, and will, therefore, draw a much larger current through it as it flows through an electrostatic field. That is, an emulsion flowing through a coalescing section entering a first electrostatic field will contain the most brine and, therefore, the first electrostatic field will draw the highest current. This current load will then determine or set the applied voltage to all the grids in known electrostatic coalescers, because there is only one power feed from a single transformer used therein. The transformers, used to supply high-voltage to the electrostatic grids in known treaters, are self-protected (reactive), and, therefore, reduce the output voltage as the current increases. Brine droplets in a wet emulsion will coalesce very easily under a weak electrostatic field, because the droplet population density is high and the space between droplets is small. Therefore, reduced voltage on a first grid does not hamper the coalescing action. However, as the emulsion travels through the coalescing section and the brine is coalesced therefrom, the reduced voltage on the subsequent electrostatic grids reduces the effectiveness of the applied electrostatic fields. The sequentially leaner emulsion traveling through the sequential grids in a treater actually requires much stronger electrostatic fields to coalesce more widely-dispersed brine droplets contained in the emulsion being treated. The emulsion flowing from a first electrostatic grid-to subsequent electrostatic grids becomes drier as water coalesces and is separated out. Therefore, in order to sustain effective coalescing at each electrostatic field, the present invention increases the voltage at each sequential grid. This is provided by using two or more transformers and power feeds to the electrostatic grids. Each electrostatic grid and transformer is operated independently, and is set at a different voltage. That is, the first stage grid will be set at a low voltage to handle wet emulsion, while the subsequent grids would be set at increasingly higher voltages to handle leaner emulsions as the emulsion travels through the coalescing section. The last field in the series will coalesce the last small quantities of water or brine remaining in the oil emulsion, by having the highest voltage applied thereto.
The present invention also has the baffles before each electrostatic grid extending all the way down to the oil/water interface in the treater section, to overcome the problem found with the ""159 patent technology. That is, wet emulsion tends to bypass the electrostatic field zone by flowing under the baffles and electrostatic grids, along the surface of the oil/water interface. By lowering the ends of the baffles to the oil/water interface, the hydraulic bypassing is reduced, and thereby increasing the efficiency and effectiveness of the present invention.
In addition, in a further embodiment of the present invention, a first distribution baffle, before the first electrostatic grid, is provided with circular holes in an offset triangular pattern with a total open area of 8%-16%. These circular holes in the triangular pattern promote more uniform horizontal plug flow of the emulsion through the treater, so that all of the emulsion is exposed to the electrostatic fields uniformly.
Therefore, it can be seen that the present invention meets an existing need in the art for the improved and more efficient treating of emulsified oil as it flows through a treater section by the use of improved electrostatic grids, the application of increasing voltage to sequential grids, together with improved and modified baffles for better controlling the flow of the emulsion through the treater.
Accordingly, it is a general object of the present invention to provide an improved apparatus for treating (dehydrating) an emulsion. It is a principle object of the present invention to provide an improved apparatus for more efficiently treating oil, gas, water emulsions flowing through a coalescing section of a treater. It is another particular object of the present invention to provide improved electrostatic elements for more effectively coalescing out droplets of brine in an oil/brine emulsion flowing through a coalescing section of a treater. It is yet another particular object of the present invention to provide an improved apparatus for achieving substantially complete separation of brine from oil by utilizing improved electrostatic elements having wing sections thereon. And, it is a further particular object of the present invention to provide an improved electrostatic coalescer having more efficient electrostatic elements, elongated baffle elements used in conjunction with such electrostatic elements, and variable voltage means to apply varying voltages to sequentially-placed electrostatic elements in a horizontal separating vessel.
These and other objects and advantages of the present invention are achieved by providing a horizontal vessel comprised of at least a coalescing section. The coalescing section has a plurality of electrostatic grids suspended therein adjacent to a plurality of baffles extending entirely across the flow path of an emulsion traveling longitudinally through the horizontal vessel. The electrostatic grid elements are comprised of wing-type grids, having a plurality of edges, which extend longitudinally along the flow path to increase the electrostatic efficiency thereof. In a second embodiment of the present invention there is provided a plurality of transformer means for providing different voltages to the sequentially-spaced grids so as to increase the efficiency thereof in separating emulsified water from the flowing oil.